1. Field of the Invention
The invention relates to improvements in near infrared quantitative analysis instruments.
2. Background of the Prior Art
Near infrared quantitative analysis instruments are known and commercially available. Such instruments as are known in the prior art make use of the phenomena that certain organic substances absorb energy in the near infrared (near-IR) region of the spectrum. In measuring the amount of energy absorbed by the substances at specific wavelengths (i.e., the Optical Density of the material) precise quantitative measurements of the constituents of the material can be determined. For example, protein, oil and moisture analyses of cereal grains can be determined by such instruments. For general introduction to near infrared quantitative analysis, see the paper presented by Robert D. Rosenthal to the 1977 Annual Meeting of American Association of Cereal Chemists, entitled "An Introduction to Near Infrared Quantitative Analysis".
Commonly owned U.S. Pat. Nos. 4,286,327 and 4,466,076 disclose near infrared instruments wherein the radiation source is a plurality of infrared emitting diodes (IREDs). In contrast to broad band light analytical instruments, IRED-type instruments are more subject to bias shift. The reason for this is that broad band light instruments generally have only a single illumination source, i.e., a light bulb, and if the illumination source changes intensity, it usually affects all emitted wavelengths identically. Thus, a change in intensity of the single light bulb does not cause bias shift of the instrument. In an IRED-type instrument with multiple IREDs, this "self compensating" effect of having a single light source does not exist. Thus, if any single IRED changes intensity, the result is an apparent absorbance that differs from the true absorbance of the sample being tested. This absorbance shift can be caused by shifts in the IRED center wavelength and changes in the IRED half-power band width. These may result from IRED temperature changes related to changes in intensity. Another source of error in IRED instruments may result from changes in the optical filters used. For example, if an optical filter changes in its band width or center wavelength, it can cause an IRED-type instrument to have a bias shift.
There thus remains a need in the art for near infrared quantitative analysis instruments that provide for reduction or elimination of error due to bias shift.